Midget thermostatic switch



y 1955 H. WEISSHEIMER ETAL MIDGET THERMOSTATIC SWITCH Filed Nov. 2, 1953estates O NHDGET THERMQSTATIC SWITCH Herbert Weissheimer Rolf Roseeu,Nurnherg, Germany, assignors to SiemensSchnekertwerkeAktiengesellschait, Erlangen, Germany, a German corpora- LionApplication November 2, @535, Serial No. 389,534 Claims priority,application Germany November 14, 1952 4 Gaines. (Cl. 20%-138) Ourinvention relates to midget-size thermostatic switches whose contactdevice comprises one or more stationary contact elements and an archedsnap plate that consists of a bimetal spring and electrically cooperateswith the stationary contact elements. More particularly,

the invention relates to thermostatic switches whose contact device isenclosed in a housing or capsule composed of a base portion and a coverportion.

Such switches are used for the protection of electric and othermachinery and must satisfy, inter alia, the following conditions: Thedimensions of the switch must be so small that it can readily beinserted between the windin turns of a machine or apparatus so that thetemperature of the winding can directly act upon the switch. The timeconstant, i. e. the delay with which the switch responds to changes intemperature, must be small and the switch must represent a substantiallyaccurate replica of the winding temperature so that the transmission oftemperature to the heat-sensitive elements of the switch is rapid andwithout appreciable losses.

As regards these conditions, the known thermostatic switches leave muchto be desired. A known thermostatic switch for the protection ofelectric motors, though satisfactorily small and suitable for mountingbetween the motor windings, does not operate with a sufliciently smalltime delay because the transfer of heat to the heatresponsive platemember of the switch is limited either to a small zone in the center ofthe plate or to the rim of the plate. Since the housing of this switchconsists entirely of metal, there is an increased temperature drop atthe mounting point of the switch as compared with normal operationwithout a built-in switch. This may result in appreciable inaccuracies.With a highly cooled electric machine of small size, for instance, itmay happen that the actual temperature of the windings to be protectedis considerably above the temperature to which the switch will respond.Another disadvantage lies in the fact that the heat-responsive diaphragmplate of the switch is freely movable within the switch housing. As aconsequence, only one-half of the deflective travel of the plate isutilized for controlling the electric circuit so that thecurrent-carrying duty of the switch is rather limited.

It is an object of our invention to provide a thermostatic switch,particularly a switch of midget size, that is suitable for theabove-mentioned protective purposes but avoids the shortcomings of theknown devices.

To this end, and in accordance with a feature of our invention, thearcuate snap plate of the switch, when in its position of rest, isseated along its entire or nearly entire surface on the base portion ofthe switch housing or capsule, and this base portion consists of a goodheatconducting material and has a face of arcuate shape engageable bythe bimetal spring plate.

According to another feature of the invention, the center point of thesnap plate is fastened to, or securely held against, the inwardly archedbase portion of the capsule. The attachment may be fixed, or it may beeffected by means of a pressure bolt that can be adall ice

justed to set the switch for different critical temperatures. Accordingto still another feature of the invention, an especially favorabledesign is obtained by making the cover portion of the capsule of athermally and electrically insulating material and mounting thestationary contact or contacts on that cover portion.

The foregoing and more specific features of the invention will beapparent from the embodiments illustrated on the drawing, in which:Figs. 1 and 2 are crosssectional views of one and the same midgetthermostatic switch in the circuit-opening position (position of rest)and in the circuit-closing position (heat-responsive position)respectively, while Fig. 3 shows a cross section of another midget-typeswitch Whose temperature of respouse is adjustable. The same referencenumerals are applied to both embodiments for designating similarelements respectively.

According to Figs. 1 and 2, the thermostatic switch has a capsulecomposed of a cover portion 1 and a base portion 2. The cover portion 1consists of thermally and electrically insulating material, forinstance, of pressuremolded plastic. The base portion 2 consists of agood heat-conducting material, for instance, copper. Each of the twoportions is generally cup-shaped. The side walls 3 and 4 of respectiveportions 1 and 2 are telescoped into one another and are firmly joinedby turning the rim of wall 4 over an annular shoulder of portion 1. Anarched snap plate 6 of bimetal and two contact rivets '7 are disposed inthe capsule space 5 enclosed by portions 1 and 2. The snap plate 6consists of a bimetal spring and has its center fastened by a rivet 6'to the base portion 2 of the capsule. The plate 6 is strip shaped, butmay also be given the shape of a star or circular disc. The base portionhas an arched cross-sectional shape bulging toward the interior of thecapsule space 5. The curvature of the snap plate and of the adjacentface of the base portion are adapted to each other so that the snapplate 6, when in the normal position of rest, is seated in the baseportion 2 along the entire surface area of the plate. The rivets 7 aresecured to the cover portion 1 of the capsule in the vicinity of the rimzone of plate 6 and are conductively connected by terminal members 3with current leads 9 on the outside of the cover portion. While only twocontact members are shown, it will be understood that three contactrivets may be provided to cooperate with respective three arms of thesnap member or with a circular snap plate.

The thermostatic switch may be mounted into the winding of an electricmachine or apparatus to be protected from overheating. When so mounted,the temperature of the winding is directly transmitted to the goodheatconducting base portion 2 of the capsule. When a given excessivetemperature is exceeded, the bimetal disc 6 snaps from the position ofrest (Fig. l) to the active position (Fig. 2) in which it touches thecontact rivets 7. This closes a control circuit which actuates a relayto re lease a circuit breaker or to issue a danger signal.

For making the switch adjustable to a desired temperature at which itwill respond, the snap plate, instead of being riveted or otherwisefirmly joined with the base portion 2, may be held against the baseportion by means of a set screw 10 that passes through the cover portion1 as is illustrated in Fig. 3. The force with which the snap plate hasits center area urged against the base portion 2 can thus be varied bychanging the setting of screw 10. In this manner, the sensitivity ofresponse of the snap plate and hence, the critical temperature can beadjusted within certain limits.

If a thermostatic switch according to the invention is to be used forthe protection of high-tension machines, the base portion of the capsuleis preferably coated with a heat conducting but electrically insulatinglayer.

zyz'ovgrss By virtue of the complete or nearly complete engagement ofthe snap plate with the heat-supplying arcuate face of the base portion,the transmission of heat by conductance is greatly improved thrusminimizing the time delay in the response of the switch. Besides, thepossible maximum of switch travel is fully utilized due to the fact thatpart of the deflectively movable plate is kept in a fixed positionrelative to the capsule. Due to the friction-free mounting of the snapplate, the switch maintains the desired critical temperature with amaximum degree of accuracy and constancy. Since the snap plate is fixedat its center point and the end or marginal Zone perform the elasticsnap movement, two or more stationary contacts can readily beaccommodated with the ad vantage of keeping the fastening means of thesnap plate free of current-carrying duty. When using heat insulat ingmaterial for the cover portion of the capsule, the heat dissipation atthe mounting point of the switch is only slight, thus preventing theactual temperature of the protected machine winding from excessivelyincreasing beyond the temperature effective upon the switch. Noadditional insulation of the stationary contacts is needed at the pointswhere they pass through the capsule. Besides, by coating the baseportion of the capsule with an electrically insulating material, theswitch is also made suitable, in a simple manner and without appreciableexpenditure, for use with machines operating at high tenston.

It will be obvious to those skilled in the art upon a study of thisdisclosure that thermostatic switches according to our invention may bemodified in various respects and may be embodied in devices other thanthose specifically illustrated and described, without departing from theessential features of the invention and within the scope of the claimsannexed hereto.

We claim:

1. A thermostatic switch, comprising a capsule having a base portion ofgood heat-conducting material and a cover portion of thermally andelectrically insulating material, an electric contact device disposedwithin said capsule and having stationary contact means mounted on saidinsulating cover portion and an arcuate snap plate lconsisting of abimetal spring and engageable with said contact means when hot, saidbase portion having a convex face facing the interior of said capsule,said plate being mounted on said base portion at the center of theplate, and said plate, when cold, having substantially its entiresurface seated upon said convex face.

2. A miget thermostatic switch, cornprisin a capsule having a baseportion and a cover portion, an electric contact device mounted in saidcapsule and having stationary contact means and an arcuate snap plate ofspringy bimetal, said base portion consisting of a good heat-conductingmaterial and having an arcuate inner face adjacent to said snap plate,said snap plate, when cold, having substantially its entire surfaceseated upon said arcuate face, and pressure means engaging said plate atthe center thereof for holding said plate against said face, saidpressure means being adjustable for varying the hold ing pressure tothereby vary the response temperature of the switch.

3. In a switch according to claim 2, said pressure means comprising ascrew mounted on said base portion and accessible from the outside ofsaid capsule for adjusting said temperature.

4. A midget thermostatic switch for use in temperature control of hightension machines, comprising a capsule having a base portion and a coverportion, said base portion having a coating consisting of a heatconducting but electrically insulating material, an electric contactdevice mounted in said capsule and having stationary contact means andan arched snap plate of springy bimetal, said base portion consisting ofa good heat-conducting material and having an arched inner face adjacentto said snap plate, and said snap plate, when cold, having substantiallyits entire surface seat upon said face.

References Cited in the tile of this patent UNiTED STATES PATENTS2,447,649 Harrison Aug. 24, 1948 2,455,011 Ingels Nov. 30, 19482,490,103 Stilwell, Ir. Dec. 6, 1949

